Various surface pretreatments have been practiced with metallic substrates such as iron, zinc plated steel, aluminium and others, to improve the corrosion resistance and coating adhesion thereof. Among the treatments proposed, particular attention is directed to, from the viewpoint of ease in operation and operational control, obviation of drain contamination, shortening of process steps and the like; and a metal surface treatment with the so-called coating type, non-rinse chromate treating agent, wherein the treating liquid containing, as main ingredients, synthetic latex and water soluble chromium compound is merely applied to the metal surface. For this end, a number of proposals have been made as, for example, Japanese Patent Application Kokai No. 57931/75, Japanese Patent Publication No. 31026/74; ibid 40865/74; ibid 1889/75 and the like. However, in heretofore proposed treating compositions, troubles have always been encountered due to the presence of surfactant or emulsifier used for the preparation of emulsion. That is, for the purpose of maintaining an emulsion in its stabilized state, employment of surfactant or emulsifier is essential but the presence of such material in the formed emulsion may inevitably cause adverse effects on adhesion, anticorrosion, water resistance of the coating prepared therefrom. Therefore, efforts have been made to solve the question of chemical stability of the emulsion without the help of the conventional type of surfactant or emulsifier and to obtain a stabilized composition comprising such emulsion and a water soluble chromium compound containing a trivalent and hexavalent Cr. In the meantime, there has been proposed a technique of using a water soluble type organic high molecular compound as an emulsifier in the preparation of resinous emulsion. That is, Japanese Patent Application Kokai No. 74934/76, describes a composition comprising a polymer emulsion and chromium compound, which is characterized in that the emulsion is prepared by using defined amounts of a particular polyacrylic acid or its ammonium salt and polymerizing .alpha.,.beta.-ethylenically unsaturated monomers, in the presence of water soluble persulfate, at a defined temperature. Also, in Japanese Patent Publication No. 39393/81, there is disclosed a metal surface treatment with the composition comprising as principal ingredients, a polymeric emulsion, a water soluble chromium compound and a water insoluble white carbon, the emulsion being prepared by emulsion polymerization of .alpha.,.beta.-monoethylenically unsaturated monomers using, as emulsifier, a specified amount of polyacrylic acid and/or acrylic copolymer. The thus obtained emulsion per se, i.e. the emulsion obtained by using a water soluble organic high molecular compound as an emulsifier, is quite stable chemically and can give a stabilized composition when compounded with chromium compound containing trivalent and hexavalent chromium and the resulting composition is very useful as a metal surface pretreating composition capable of forming a pretreatment film with excellent corrosion resistance and adhesion properties. However, in considering a surface pretreatment of metallic substrate, it is of great importance to take into account, besides the stationary adhesion between the substrate and the composition, the coating adhesion under bending and processing conditions, as well as the scratch resistance of the coating. Generally speaking, the properties of film adhesion under bending condition and of scratch resistance conflict with each other. In the aforesaid Japanese Patent Application Kokai No. 74934/76, attention is only directed to the adhesion under processing condition and not at all to the scratch resistance. And, in Japanese Patent Publication No. 39393/81, the claimed effects are merely of the interaction of the disclosed three components, i.e. emulsion, water soluble chromium compound and water insoluble white carbon, and no statements are given as to the scratch resistance and bending behavior in the two component system as in the present invention. Furthermore, even in the said three component system, the effects of scratch resistance and bending resistance fluctuate considerably and it was found that good results were not always obtained therewith. Especially, a low temperature bending will often cause troublesome adhesion failure and there is a trend that the greater the content of the trivalent chromium compound, the lesser the storage stability of the liquid concentrate.
The inventors, have made studies for the purpose of obtaining a metal surface treating composition comprising as main ingredients, an emulsion and chromium compound, the emulsion per se being stable irrespective of containing no surfactant and capable of giving a stabilized composition with chromium compound, which is excellent in storage stability in its concentrated form and which can result in a film with excellent anticorrosive and water resistant properties and moreover, which has improved scratch resistance and well balanced bending properties at a room temperature and a lower temperature. They have found an anticorrosive metal surface pretreating composition having improved bending and scratch resistant properties, comprising as essential components an aqueous emulsion of hard polymer microparticles obtained by emulsion polymerization of .alpha.,.beta.-ethylenically unsaturated monomers, using as an emulsifier, polyacrylic acid and/or copolymer of acrylic acid with at least one member selected from the group consisting of methacrylic acid, acrylamides, methacrylamides and hydrophilic monomers of the formula: ##STR1## in which A stands for a hydrogen atom or a methyl group, R represents a substituted or unsubstituted alkylene having 2 to 4 carbon atoms, and X is a functional group containing at least one of oxygen, phosphorous and sulfur atoms, in an amount, on the solid basis, of 5 to 100 parts by weight per 100 parts by weight of said monomers, the polymer microparticles having a mean diameter of 0.1 to 3.mu. and being hardened by using as a part of said .alpha.,.beta.-ethylenically unsaturated monomers, either monomer having in its molecule two or more ethylenic unsaturation bonds capable of entering into a radical polymerization, thereby causing inner geleation of the polymer, or appropriate monomers capable of bringing the glass transition temperature of the polymer to 15.degree..about.110.degree. C., and a water soluble chromium compound, 30 to 50% by weight of whose chromium are trivalent, the weight ratio of emulsion solid to metal chromium being 2:1 to 5:1. On the basis of said finding, the inventors have applied for a patent (Japanese Patent Application No. 92663/82).
The abovesaid invention had been made, starting from the theoretical considerations such that since a metal surface is full of microscopic undulations if a device is made on the resinous microparticles so as to have a mean diameter that is small enough to be set into the depressed portions of said undulations and to be of a considerably hard nature, then the resistance to shear stress will be increased, which must be of use in stress relaxation in the vicinity of said particles. Thus, the said invention intends to provide a non-rinse coating type chromate treating composition capable of resulting in a coating with excellent anticorrosive, bending and scratch resistant properties and the like. However, such metal surface treating composition is generally applied onto a metal substrate whose surface has already been treated with zinc phosphate or the like. Therefore, at the time when the said composition is applied by a conventional means as spraying, dipping or roll coating, there is often observed a phenomenon in which an amount of metal ions from the undercoat is accumulated in the treating composition by the recycle of a part of the coating liquid and if the employed composition is based on an emulsion containing polyacrylic acid or the like, thus accumulated metal ions and said polyacrylic acid react with each other, thereby forming the corresponding salt and causing an increase in viscosity or inner gelation of the treating composition. Therefore, it would be an inmeasurable advancement of the technical level in the relevant industry to provide a metal surface pretreating composition comprising as main ingredients, an aqueous emulsion and chromium compound, which is stable without the inclusion of surfactant and capable of resulting in a coating with excellent bending and coin-scratch resistant properties, and containing no materials such as acrylic copolymer which may cause, through reaction with metal ions, an increase in viscosity or inner gelation of the composition. The present invention has been made to solve the abovesaid problems.